The present invention relates to measurement apparatus for measuring the emittance of small diameter beams, e.g., charged particle beams, of the type found in accelerators and beam transport systems.
Various types of accelerators are currently available for accelerating charged particles, e.g., atomic sized particles (ions), to very high velocities. At high velocities, such particles exhibit significant kinetic energy and may be considered as a "beam" that can be used advantageously for research, medical, industrial or military applications.
An emittance scanner is a measurement tool or device used to characterize the quality of a charged particle beam. More particularly, an emittance scanner measures the "emittance" of a beam, defined below. As its name suggests, an emittance scanner scans a cross section of the beam in order to characterize the beam at each point in the beam.
To better understand the concept of beam "emittance", reference is made to FIG. 1 where there is shown a schematic representation of a diverging beam 12. (It is noted, of course, that not all beams are diverging. The diverging beam of FIG. 1 is merely cited as an example of one type of beam.) Because a beam of charged particles is comprised of many individual charged particles, each of which may follow a somewhat different path, the beam 12 is schematically represented in FIG. 1 as several lines. Each line may be considered as the path followed by a different charged particle within the beam. Using the XYZ coordinate system shown in FIG. 1, the beam 12 is generally moving in the Z direction. Because a diverging beam is represented, the lines representing the beam are grouped closer together at a first point z.sub.1 along the Z-axis than they are at a second point z.sub.2 along the Z-axis, where z.sub.2 &gt;z.sub.1, i.e., the first point z.sub.1 is upstream from the second point z.sub.2 .
At any point along the beam axis z, it is useful to examine the beam and characterize each point of the beam in the cross section, i.e., each point in the X-Y plane, by an angle measurement that indicates whether a beam particle at that particular point of the scanned cross section is moving away from or towards the Z-axis. These angular deviations from the Z-axis are typically represented by the rates of change of the transverse coordinates (x and y) with respect to z, or x'=dx/dz and y'=dy/dz. The ensemble of points representing the beam occupy a certain region or regions in the four-dimensional phase space, x-x'-y-y'. Projections of this region on the x-x' and y-y' phase planes represent the common two-dimensional descriptions of the beam. Such two-dimensional descriptions may be plotted for each axis in an X-Y plane, with one axis (e.g., the horizontal axis) of the plot being the scan axis, and the other axis (e.g., the vertical axis) being the measured angle. For a beam that is diverging (expanding) as represented in FIG. 1, such a plot of points (obtained by scanning the beam along the X-axis) may appear as shown in FIG. 2, with the shaded area A.sub.X representing the distribution of the data points thus measured. (A similar plot may be made for scanning along the Y-axis, resulting in a similar area A.sub.Y of the distribution of data points thus measured.) Such a plot is referred to as an emittance plot, and the data thus obtained may be referred to as an emittance plane. There are thus two emittance planes, one resulting from scanning the beam cross section in the X-axis direction, the other by scanning in the Y-axis direction.
The area A for an X-emittance plane (and a similar area A.sub.Y for a Y-emittance plane) may be considered as the area occupied by the beam. This area, divided by .pi., is numerically defined as the "emittance" of the beam for that particular emittance plane. It is noted that the beam area, as plotted in an emittance plane such as shown in FIG. 2, will always be constant for a given beam at all points along the beam path providing there are no beam losses. However, the shape and location of the data points plotted on the emittance plane will vary significantly depending upon the characteristics of the beam at the point in the beam path where the emittance measurement is made. Thus, it is the shape and location of the beam area (e.g., A.sub.X or A.sub.Y) on the emittance plane that provide the most useful information in characterizing a given beam.
Thus, the areas of the two-dimensional projections and the volume of the four-dimensional region constitute important measures of the quality of the beam, as these measures remain relatively unchanged in the course of beam transport. However, the orientation of these areas and volumes change during beam transport. The objective of a beam emittance measurement is thus to measure the areas, volumes and orientations of these two, three, and four-dimensional phase spaces.
For a more thorough description of beam emittance, and the techniques known in the art for measuring beam emittance, see, e.g., Steffan, K. G., High Energy Beam Optics (Interscience, New York 1965).
One of the more common techniques known in the art for measuring beam emittance is to use a mini-scanner 22 such as is schematically depicted in FIG. 3. The mini-scanner 22 comprises a box 23, typically about 4 inches in length, having a front face 24 with a small aperture or slit 26 therein. The front face 24 is designed to intercept a beam 20. All but a very small portion of the beam 20 is absorbed by the front face 24 of the scanner 22. A small portion of the beam 27 passes through the aperture 26 and is detected by a linear collector 28. The collector 28 includes appropriate electronic circuitry for generating a detection signal, represented in FIG. 3 by the signal waveform 29, that indicates the relative location on the front of the collector 28 where the beam portion 27 is detected. This location, relative to the fixed reference location of the aperture 26, thus provides a measure of the angle of the beam portion 27 within the beam 20. The entire scanner 22 is scanned through the cross section of the beam 20 in the direction indicated by the arrow 30. Typically, suitable mechanical means are employed to position the scanner 22 at N stops as it is scanned through the beam 20, with a separate measurement being made at each stop. The scan direction is first made in one direction, e.g., the X-axis direction, to enable an X-axis emittance plot to be generated. Then, as required, the measurement is repeated by scanning in the Y-axis direction, to enable a Y-axis emittance plot to be made. Often, two separate mini-scanners are employed, one to measure the X-emittances and the other to measure the Y-emittances.
The prior art mini scanner 22 described above is advantageously simple in construction, small in size, and correspondingly modest in cost. As a result, it has enjoyed widespread use and has been accepted as an international standard for beam emittance measurement. However, there are numerous kinds and types of measurements that are not possible with such a scanner.
For example, an "on-line" measurement of beam emittance, i.e. a measurement of the beam emittance without significantly interrupting or disturbing the beam (so that the beam can continue to be used for a particular application) is not possible. This is because an emittance measurement made with the mini scanner 22 is totally (100%) destructive of the measured beam because the scanner completely blocks the beam path. Due to the complete interruption of the beam, the beam is thus not available for any other purpose while the emittance measurement is being made. Further, tandem measurements of the beam emittance are not possible (i.e., simultaneous beam emittance measurements at different locations along the beam path). Moreover, the beam emittance measurement measures only one emittance plane (the plane obtained by scanning the X-axis, or the plane obtained by scanning the Y-axis), and hence a measurement of both emittance planes requires two separate measurements and is thus somewhat labor intensive.
Also, it is known that use of the mini-scanner has some effect on beam neutralization. That is, insertion of the scanner 22 into the beam path 20 perturbs to some degree the true characteristics of the beam. Thus, there is some degree of uncertainty always present in any beam emittance measurement made with such a device. Further, the device absorbs all of the beam energy, and thus some provision must be made to remove the heat resulting from such absorbed energy, as well as to handle any radiation that may be present from the absorbed beam.
In view of the above, it is evident that what is needed is a new beam emittance measuring device that offers all the advantages of the prior art mini scanner (e.g., small in size and modest in cost), but that also allows on-line measurements, intercepting only a very small percentage of the beam; allows tandem measurements; and readily measures both emittance planes. Further, such a new device would preferably not neutralize a charged particle beam, and would absorb minimal beam energy. The present invention advantageously provides a beam emittance measurement device that addresses these and other needs.